Elimination of floating slime during electrolytic refining of copper

ABSTRACT

A METHOD FOR ELIMINATING FLOATING SLIME IN THE ELECTROLYTIC REFINEMENT OF COPPER. THE METHOD USES A NOVEL ELECTRROLYTE WHICH CONTAINS SPECIFIC QUANTITIES OF TRIVALENT ARSENIC, PENTAVALENT ARSENIC AND PENTAVALENT ANTIMONY, THE ARSENIC CONTENT BEING OBTAINED ANDD MAINTAINED BY SPECIAL PROCEDURAL STEPS SUCH AS DIRECT ADDITION OF THE SUBSTANCES TO THE SOLUTION, REDUCING OF EXISTING QUANTITIES OF AS (V) IN THE SOLUTION OR BY INCREASING THE ARSENIC CONTENT IN THE ANODE. THE METHOD ALSO INCLUDES THE EXPEDIENT OF INCREASING THE CURRENT DENSITY OF THE ELECTROLYSIS, BY USING THE NOVEL ELECTROLYTE IN COMBINATION WITH PERIODIC REVERSAL OF THE CURRENT FLOW.

N. F. R. LINDSTROM 3,753,877 ELJMINATION OF FLOATING SLIME DURING ELECTROLYTIC Aug. 21, 1973 REFINING OF COPPER Filed May 25. 1971 United States Patent Ofiice 3,753,877 ELIMINATION OF FLOATING SLIME DURING ELECTROLYTIC REFINING OF COPPER Nils Folke Rune Lindstrom, Skelleftehamn, Sweden, assignor to Boliden Aktiebolag, Stockholm, Sweden Filed May 25, 1971, Ser. No. 146,663

Claims priority, application Sweden, May 28, 1970,

Int. Cl. B011; 3/06; C2211 N16 US. Cl. 204-406 19 Claims ABSTRACT OF THE DISCLOSURE A method for eliminating floating slime in the electrolytic refinement of copper. The method uses a novel electrolyte which contains specific quantities of trivalent arsenic, pentavalent arsenic and pentavalent antimony; the arsenic content being obtained and maintained by special procedural steps such as direct addition of the substances to the solution, reduction of existing quantities of As(V) in the solution or by increasing the arsenic content in the anode. The method also includes the expedient of increasing the current density of the electrolysis, by using the novel electrolyte in combination with periodic reversal of the current flow.

The present invention relates to a method for eliminating floating slime when refining copper electrolytically with copper anodes which are contaminated with antimony and/or bismuth.

Copper refining by electrolysis is efiected between an anode and a cathode in an electrolyte comprising an aqueous solution of copper sulphate, the copper content normally being 35-50 g./l., and in the presence of approximately 150-250 g./l. sulphuric acid, which increases the electrical conductivity of the electrolyte. It is known that arsenic together with antimony and bismuth in solution forms so-called floating slime, which is deleterious to the current efliciency and the cathode quality.

The electrolysis is normally carried out at temperatures between 55 and 65 C. At temperatures higher than 65 C. the amount of water evaporated from the electrolyte is extremely high, which ultimately results in higher heating costs. Moreover, at such high temperatures the cathod structure and therewith the quality of the cathode copper is impaired. At temperatures below 55 C. there is a risk of anode passivation.

The anode normally contains from 98 to 99.5% copper.

When the anode is dissolved electrolytically, metals of more noble character than copper, such as silver, gold and the platina metals are insoluble and together with other insoluble impurities in the copper, such as selenides and tellurides, form a grayish-black to black slime which is initially built up as a 0.5-1 cm. thick coating on the anode surface but which gradually loosens and sinks to the bottom of the electrolytic cell. The layer of slime on the anode, so-called anode slime, also contains finely divided, copper powder due to the fact that the anode contains oxygen. The oxygen in the anode copper is present, inter alia, as copper(I)oxide, Cu O. Copper(I)oxide is dissolved in the electrolyte according to the reaction Patented Aug. 21, 1973 Cu O+H SO CuSO -l-H 'O+Cu because at equilibrium Cu ++Cu 2Cu+ the reaction is considerably moved to the left. The copper powder formed is incorporated in the anode slime. Copper powder is also formed in the anode slime owing to the fact that the reaction Cu- Cu ++2e is slower than the reaction Cu Cu+ +e'". in this way a surplus of Cu' -ions is formed adjacent the anode surface, which ions, in accordance with the above equilibrium, are later disproportioned to Cu -ions and finely divided copper.

The lead present in the system passes primarily into solution as Pb -ions, but is immediately precipitated as PbSO and follows the anode slime.

The tin present in the system primarily passes into solution as Sn +-ions, but is then precipitated as tin(IV)hydroxide in gel form and also follows the anode slime.

The nickel and arsenic present in the system pass practically completely into solution.

The Ni +-ions are not precipitated and the concentration of Ni +-ions in the electrolyte would increase if the electrolyte were not drained OE and replaced with freshly produced nickel free electrolyte. By removing a quantity of saturated electrolyte and replenshing with fresh electrolyte, the contamination of nickel in the cathode can be maintained at an acceptably low level, as this contamination normally being solely in the form of electrolyte inclusions. The drained electrolyte can be evaporated and the nickel sulphate and the copper sulphate can be recovered. Residual copper in the drained electrolyte is then recovered by electrolysis using insoluble anodes. The presence of nickel-ions in the solution also reduces the solubility of the copper sulphate and lowers the electrical conductivity of the electrolyte.

The amount of arsenic contained by the anode normally exceeds the quantity required stoichiometrically for precipitating antimony and bismuth, and hence draining off the electrolyte also assists in regulating the arsenic content in the solution. Draining of the electrolyte also serves to maintain a constant content of copper in the electrolyte. For example, if no electrolyte were drained off, the copper content of the electrolyte would increase as a result of the aforementioned reaction and because oxygen dissolved in the electrolyte oxidizes the finely divided copper powder contained in the anode slime and the electrodes which consist of copper, whereupon copper(II)ions pass into solution. The fresh electrolyte which is used to replace the drained electrolyte should therefore be free from nickel and arsenic and have a low copper content. It is often an advantage to use an electrolyte which comprises solely diluted sulphuric acid. The copper content in the electrolyte can also be kept at a constant level by introducing insoluble anodes, for instance lead, in one or more cells. Thus, it is possible to reduce the copper content without increasing the drainage. In this way it is possible to keep a high arsenic content in the electrolyte without taking further measures. Soluble oxygen in the electrolyte also oxidizes the three valent arsenic and three valent antimony present to the five valent state. It is presumed that Cu(I) ions catalize this oxidation.

Practically all the bismuth present passes into solution in trivalent form and is not oxidized by atmospheric oxygen dissolved in the electrolyte.

During the electrolytic process a drop in voltage is obtained owing to an ohmic resistance in the busbars, electrical contacts and the ohmic resistance in the electrolyte, together with so-called activation polarisation caused by the resistance transforming copper ions in the electrolyte to copper atoms in the metal lattice and vice versa, and so-called concentration polarisation, caused by differences in ion concentration adjacent the electrodes in relation to the concentration in the main body of the electrolyte. Movement of the copper ions between the anode and the cathode takes place mainly through the process of convection and diffusion, while current transport is mainly eifected with hydrogen ions. Thus a thin layer of electrolyte is formed around the surfaces of the electrodes having a composition ditferent to the main body of the electrolyte. Circulation in a conventional electrolysis cell cannot affect the composition of these films to any appreciable extent. The cathode film becomes deplete of copper ions and slightly enriched with sulphuric acid, and consequently the density becomes lower than the density of the main 'body of the electrolyte, whereby the depleted electrolyte flows upwards along the cathode. In turn, the anode film is enriched with copper ions and slightly depleted of sulphuric acid and thereby obtains a greater density than that of the main body of electrolyte, and hence the anode film enriched with copper sulphate flows downwards along the anode. This behavior of the film causes an enrichment of copper sulphate in the lower parts of the electrolytic cell. To avoid an uneven result, this must be counteracted by some form of vertical circulation of the electrolyte through the cell. A vertical circulation can be achieved by removing the electrolyte either from the upper part of the cell and admitting electrolyte to the lower part or vice versa, at the same time as a certain horizontal circulation is efiected by admitting the electrolyte to one end of the cell and removing it from the other. The differences in the concentration of the copper sulphate between the upper part and the lower part of the cell is more effectively equilized with increasing circulation rate. On the other hand, at high circulation rates there is a risk of the anode slime swirling. A normal circulation rate is -20 l./rnin. through an electrolysis tank containing 5000 l.

The amount of energy consumed per unit of precipitated copper increases with increasing current density. The fact that with an increased current density it is possible to increase the production rate and obtain a more rapid run of material is naturally an economic advantage. In general, the consumption of energy has small economical significance in comparison with the capital costs, and therefore most plants operate with the highest possible current density, normally 200-270 a./rn.

In order to obtain a high quality of the cathode copper, it is necessary to use different organic additives. Glue, thiocarbamides, goulac are normally used in this respect.

With technical electrolysis the size of the cathode and anode is normally selected on the basis of a compromise made between difierent economical and technical viewpoints, and is generally approximately 1 x 1 m. In certain instances the electrolysis process is carried out with each electrode in series, whereby one side of the electrode serves as a cathode and the other side as an anode. As a rule, however, all cathodes and anodes of each cell are connected in parallel. The distance between the centre lines of the anodes is normally 9-13 cm. and the cathodes are placed centrally therebetween.

With increasing distance between the electrodes the risk of falling slime from the anodes contaminating the cathodes decreases and, at the same time, a better cathode structure is obtained. On the other hand, plant. costs increase with increased electrode spacing.

When producing electrolytic copper, a floating slime is normally formed whichconsists of an oxide complex of bismuth, arsenic and antimony in proportions which are dependent on the concentration of these substances in the electrolyte. This concentration depends primarily on the contents of said elements in the anodes. Floating slime shows very small tendency to settle, remains suspended in the electrolyte and impurifies the cathode copper and causes growths to form on the cathode, which can result in short circuiting between the anode and the cathode and thereby reduce the current efliciency. Moreover, growths on the cathode act as settling surfaces for the anode slime.

Despite the fact that a large number of works have been published in which attempts have been made to establish the reasons why floating slime is formed, no one has hitherto been able to find the casual connection. Among others, Livshits and Pazukin (J. Applied Chemistry of the U.S.S.R. 27 (1954), pages 283292) have carried out a comprehensive examination and reached the conclusion that the only way in which floating slime can be avoided is to reduce the antimony content in the anodes to an extent such that the content of antimony in the electrolye lies beneah 0.5 g./l. G. Graf and A. Lange also discuss in Neue Hiitte 10 (1965), pages 2'l6220, the reasons for the formation of floating slime and report that this depends on the formation of antimony arsenate in amorphous form. Graf and Lange also recommend a restricted amount of antimony in the anode. None of these publications indicate or show a method for producing high grade cathode copper from anodes with high content of antimony.

It has been assumed in the aforementioned publications that the floating slime derives from the precipitation of trivalent antimony with pentavalent arsenic, the latter being oxidized by oxygen which is dissolved in the electrolyte. According to Graf and his colleagues, trivalent antimony is oxidized to pentavalent antimony and with sufficiently high contents is also precipitated as Sb O The oxidation of antimonyflll) and arsenic(III) to pentavalent ions in the absence of a catalyst is extremely slow, but, as previously mentioned, the presence of monovalent copper is considered to catalyse the oxidation by forming 0 according to the reaction When the antimony content of the anode is greater than approximately 400 g./t. and/or the bismuth content of the anode is greater than approximately 200 g./t. it has not previously been possible to efiectuate a copper electrolysis obtaining a high grade cathode copper. As the quality and purity demands placed on the cathode copper is continually increasing the contaminants which deleteriously affect the quality properties of the copper must be maintained as low as possible. Antimony and/or bismuth strongly impair the quality of cathode copper. Normally, the quality of the cathode is valued by determining the soft annealing temperature (recrystallisation temperature). Even relatively small contents of antimony and/or bismuth will result in an excessively high soft annealing temperature, which renders the product less suitable for important fields of use, particularly such fields as the manufacture of fine wire filamerits etc. It is true that the major content of the antimony can be removed during the processing of the copper raw materials to anode copper but it is still very dilficult and expensive to attain the desired low antimony content. It is still more diificult to remove bismuth during these treatment stages. Consequently, it has hitherto been impossible to utilize raw copper material with high antimony and/ or bismuth content for producing high grade electrolytic copper.

DESCRIPTION OF THE INVENTION The present invention is concerned with a method for electrolytically refining copper anodes which contain more than 200 grams of antimony per ton and/or more than grams of bismuth per ton without getting floating slime, and is characterized by the steps of regulating the quantities of trivalent arsenic, pentavalent arsenic and pentavalent antimony in the electrolyte so that in the steady state the content of As(1l) exceeds 1 g./l., the content of As(V) exceeds 2 g./l. and the quantity of Sb(V) is less than 0.05 g./l.

Thus, in accordance with the invention trivalent arsenic can be introduced to the electrolyte in quantities at which the content of trivalent arsenic at the steady state is maintained above 1 g./ 1., preferably between 25 g./l. The trivalent arsenic present in the system will absorb oxygen dissolved in the electrolyte, the arsenic being, at the same time oxidized to pentavalent arsenic. At the same time, the electrolyte shall contain at the steady state at least 2 g. of As(V) /l. preferably 7-15 g./l., causing crystalline arsenates of trivalent antimony and bismuth to precipitate. In the case of anodes which do not contain antimony but which do contain bismuth, the content of pentavalent arsenic is of greater significance than the content of trivalent arsenic.

The increase of AS(HI)-content of the electrolyte can be caused by dissolving a suitable As(III) compound, for example AS203, in the electrolyte or by adding an aqueous solution of AS203 thereto. It is also possible to admit arsenic to the electrolyte via the anodes, from which it passes into solution as trivalent arsenic.

The present invention thus includes a method for completely avoiding the formation of flotation slime in the case of anodes which contain high contents of antimony, and it has been discovered that anodes having, for example, up to 2500 grams of antimony per ton can be used successfully. Furthermore, the present invention solves the problems which occur when the anodes contain large quantities of bismuth, for example, up to 2500- g./t. Theoretically, anodes which have higher contents of antimony and particularly bismuth can be used since in respect of the formation of floating slime no upper limit with regard to the antimony and bismuth contents has been observed.

The mechanism by which floating slime is formed has now been established, and above all the key role played by pentavalent antimony. It has namely been discovered that antimony and arsenic pass into solution in trivalent form and that further oxidation to pentavalent form is primarily catalysed by elementary copper. In the conventional electrolysis of copper which contains antimony a certain amount of pentavalent antimony is always formed as a result of oxidation of the antimony with atmospheric oxygen which to some extent is dissolved in the electrolyte. This amount is normally of the order of 0.050.20 g./l., while in the presence of antimony(V) non-settling amorphous floating slime precipitates are obtained. These precipitates are chemically non-defined compounds existing between pentavalent antimony, trivalent antimony, trivalent bismuth (when present), pentavalent arsenic, oxygen and water. Trivalent arsenic is not included in the floating slime. Foating slime gives no defraction lines when subjected to X-ray defraction analysis. The composition of the compounds defining floating slime varies with the concentration of the components in the electrolyte, although the atomic relationship between arsenic and the sum of antimony and bismuth normally lies between 1:15 and 1:2. It is probable that the presence of elementary copper is necessary for the oxidation of arsenic(IIl) and antimony (III), since peroxides are formed on the surface of the copper, which in turn oxidize arsenic(III) and antimony(III). It has been discovered that the rate of oxidation is higher with the oxidation of arsenic(ll1) than with the oxidation of antimony(III), and hence arsenic- (III) primarily absorbs the oxygen dissolved in the electrolyte. It has been established that pentavalent antimony is deleterious to the copper electrolysis not only because it contributes to the formation of floating slime but also because it retards the precipitation of antimony and/or bismuth arsenates. A conventional electrolyte thus becomes highly oversaturated with respect to bismuth and antimony.

It has also been established that if no pentavalent antimony is present or if any only present in quantities below approximately 0.05 g./1. electrolyte, only crystalline precipitates of antimony arsenate and/or bismuth arsenate are formed and no floating slime. X-ray defraction analysis of the precipitates clearly shows lines of well-formed crystalline phases. The atomic relationship between arsenic and the sum of antimony and bismuth is always equal to one. Antimony arsenate is a defined chemical compound which has been found to have monoclinic crystal modi fication with the edge length of 12:5.318 Angstroms, b=6.917 Angstroms and 0:4.814 Angstroms and the monoclinic angle=93 The composition is probably SbOH AsO Similarly, bismuth arsenate is a defined compound which occurs in two crystal modifications, tetragonal (the stable) of monoclinic, with the composition BiAsO The crystalline precipitates settle readily and behave principally in the same manner as, for example, PbSO and sink to the bottom of the tank together with other anode slime.

The method of the present invention thus prevents the formation of pentavalent antimony to an extent such that the concentration in the solution is maintained below 0.05 g./l., preferably below 0.02 g./l. In this way, formation of the previously described floating slime is rendered impossible and crystalline antimony arsenate and bismuth arsenate precipitate onto graft crystals present in the anode slime. If minor quantities of pentavalent antimony are formed these are removed by co-crystallisation. The crystallisation also causes the total contents of antimony and bismuth in the electrolyte to be greatly reduced.

In the addition to the use of trivalent arsenic, as a mean of preventing the formation of pentavalent antimony, oxidation of trivalent antimony by ambient air can be prevented by excluding contact of the electrolyte and atmospheric air by means of air tight pumps and an air tight circulation system, for example. The content of pentavalent antimony can be maintained in this way below 0.05 g./l. It is also possible to reduce the quantity of oxygen dissolved in the solution by using an inert gas to protect the circulation system. The electrolytic cells can be protected against atmospheric oxygen by known expedients devised for the purpose of reducing evaporation of the electrolyte surface, for example expedients as covering the electrolyte with buoyant plastic bodies, plastic cloths or an oil layer.

As will be apparent from the aforegoing the formation of floating slime in the electrolytic refinement of copper anodes which contain more than 200 g./ t. antimony and/ or more than g./t. bismuth can be avoided by maintaining the content of pentavalent antimony in the electrolyte at the steady state beneath 0.05 g./l. and by maintaining the content of pentavalent arsenic above 2 g./l. The content of trivalent arsenic should be less than 1 g./l., preferably 25 g./l. This can be attained by ensuring that the total quantity of arsenic in the electrolyte is sufficiently large to fulfill the aforementioned conditions, which can be effected by adding primarily trivalent arsenic to the electrolyte, the arsenic absorbing the oxygen dissolved in the electrolyte and thereby preventing primary dissolved trivalent antimony from being oxidized to pentavalent antimony.

It has also been discovered experimentally that the formation of floating slime can be prevented during the electrolytic refining of copper if in an electrolyte containing totally at least 3 g./l. arsenic, preferably 9-20 g./l., a such large quantity of pentalavent arsenic in the electrolyte is reduced by the action of sulphurdioxide that the quantity of trivalent arsenic in the electrolyte is at least 1 g./l., preferably 2-5 g./l. at the steady state.

The sulphur dioxide is suitably added to the electrolyte in liquid form or in gas form and can be charged to the system through suitable metering devices, such as nozzles, and this is suitably admitted at the lower part of the electrolytic cell.

With a particularly preferred embodiment of one aspect of the invention sulphur dioxide is supplied to a connected absorption apparatus through which a small portion of the electrolyte is circulated. As will be evident from the examples given below, only relatively small quantities of 50 are required, in the majority of cases less than 1 ton of sulphur dioxide per 1000 tons of copper produced.

In accordance with the invention, the absorption column used to introduce sulphur dioxide into the diverted portion of the electrolyte stream can be followed by a stripper adapted to remove completely all 50 residues so that an odourless electrolyte is obtained. It is normally necessary to supply heat to the stripper in order to prevent the electrolyte from being cooled by evaporation with subsequent risks of copper sulphate crystallising out. However, in order to render it unnecessary to heat the electrolyte to boiling point in order to strip the sulphur dioxide, an inert gas can be passed through the stripper. The inert gas used can be air, and since neither metallic copper nor any other catalyst is present the trivalent arsenic or trivalent antimony is not oxidized to the corresponding pentavalent state. In the case of using inert gas for stripping you might prevent cooling of the electrolyte by adding steam.

It has also been discovered that when using the new electrolyte described above it is possible by reversing the direction of the current at periods of the order of l-% of the total electrolysis period to avoid both the formation of floating slime and anode passivation at higher current densities than would otherwise have been possible with anodes containing 500 g./t. antimony or thereabove. Consequently, anode passivation is now no longer a determining factor in setting the limit of the maximum possible current density. It has been discovered that anode passivation can be entirely avoided up to the current density limit set by the level of the impurities on the cathode copper.

The mechanism of anode passivation has been established in connection with the work carried out in developing the present invention. The anode slime present on the anode in the form of a thick coating renders it difficult for copper sulphate to diliuse from solution adjacent the anode surface out into the main body of the electrolyte. This causes the concentration of copper sulphate nearest the anode surface to be of such magnitude as to exceed the solubility limit of copper sulphate. Copper sulphate crystals then precipitate in the anode film and block the passage of the current. This blocking effect leads to an uneven dissolution of the anode and cavities and holes occur by the side of non-electrolytically dissolved areas.

It has previously been assumed that the reason for anode passivation is the formation of oxides, but this assumption has now been shown to be false. The anode copper surface, which in a normal electrolysis is rough, binds the anode slime to an extent whereby a layer of slime from one half to one cm. (/2-1 cm.) thick is formed on the anode. In the case of passivation the surface of the anode copper is polished by the electrolyte and the slime is unable to adhere satisfactorily to the surface of the anode copper. The slime thus falls down and eddies in the electrolyte, which results in a considerable increase of slime enclosures in the cathodes. Anode passivation also leads to an increase in the anode potential with increased energy consumption as a result, and an increased electrochemical dissolution of silver ions, which are electrolytically precipitated onto the cathode.

With the conventional electrolysis of antimony and hismuth containing anode, the gel-like floating slime acts to seal off the anode slime coating. Thus, anode passivation takes place at a lower current density than with an electrolysis using the same anodes but with an electrolyte pro* duced in accordance with the present invention.

The tendency of anode passivation as a result of the sealing eifect of the gel-like floating slime on the anode slime layer has been found to increase considerably with increasing contents of antimony in the anodes, and consequently such passivation is normally designated antimony passivation.

The present invention eliminates the sealing floating slime in the anode film. In this way, the diffusion of copper ions through the anode film is facilitated and hence a higher copper ion concentration in the electrolyte can be permitted without the phenomenon of anode passivation taking place. This involves a corresponding increase in the copper ion concentration at the cathode, which is of particular importance with respect to improving the quality of the cathode copper at high current densities.

When increasing the current density, for example by current reversal, the intensity of the critical impurities antimony and bismuth is also increased. In conventional electrolyte, this causes an increased impurity pressure in the electrolyte and therewith increased infection of floating slime on the cathode. Irrespective of what takes place at the anode, this sets a limit to the extent to which it is possible to increase current density without impairing the quality of the cathode. The present invention also eliminates this limit with respect to the increase of current density.

Furthermore, it is possible when using this method and while retaining the cathode quality to work at higher electrolysis temperatures than would otherwise have been possible. In a conventional electrolyte the higher temperature causes more rapid oxidation of trivalent antimony to pentavalent antimony, and normally an intensified formation of floating slime is obtained. By using a combination between the current reversal principal and the electrolyte according to the present invention the deleterious floating slime formation can be fully eliminated. A high electrolyte temperature, in excess of 60 C., impairs the efiect of the inhibitors, although by increasing the quantities of the inhibitors the cathode quality is maintained or even improved. In turn, the increase in temperature means that the current density can be further increased, owing to the lower viscosity and higher electrical conductivity of the electrolyte and the higher solubility degree for cop per sulphate.

The invention will now be illustrated by way of a number of examples.

EXAMPLE 1 The experiment was carried out on factory scale and the electrolyte partially protected against oxidation by ambient air by using an air tight circulation system. As(III) was supplied to the system as a reduction agent.

A conventional electrolyte comprising 40 g./l. Cu, 170 g./l. H 4 g./l. As, of which 0.5 g./l. Was Asflll), 0.43 g./l. Sb of which 0.10 g./l. Sb(V) and 0.35 g./l. Bi was used during the initial stage of the experiment. This represents a conventional electrolyte at the steady state.

Trivalent arsenic was added in the form of an aqueous solution containing 30 g./l. AS203 (saturated at room temperature). During the first two weeks, an aqueous solution containing 60% A3205 was also added. The anodes comprised an anode copper having 98% Cu, 0.40% Ni, 0.40% Ag, 0.11% As, 0.035% Sb and 0.020% Bi.

The experiment was conducted in 28 conventional copper electrolytic cells in commercial operation. The steady state was reached after three weeks, the contents of the electrolyte being approximately 4 g./l. As(III) and approximately 11 g./l. As(V). The total antimony content stabilized at less than 0.2 g./l. of which was Sb(V) less than 0.02 g./l., and the bismuth content at less than 0.15 g./l. During the running-in time, cathodes of normal quality were initially obtained. The quality of the cathodes was progressively improved, however, owing to the reduced impurities of antimony and bismuth in the cathodes.

Subsequent to reaching the steady state, the original anodes, which had an antimony content of 350 g./t., were replaced with anodes having an antimony content of 800 g./t. The good quality of the cathodes was unaifected by the high antimony content of the anodes owing to the fact that no floating slime was formed.

Ag Fe 7 Ni 1.5

As 0.5 Te 0.1 Se 04 S 7 Analysis made on wire bars manufactured from cathodes obtained from a conventional electrolysis process with only 350 g./t. antimony in anodes gave the following result:

Ag 10 Fe 7 Ni 1.5 Pb 1.5 Bi 0.3 Sb 0.6 As 0.5 Te 0.1 Se 0.4 S 7 EXAMPLE 2 The experiments were carried out in four factory scale conventional copper electrolysis cells in commercial operation and provided with separate circulation systems. In the first run the anodes had the following impurities among others: 0.15% Ni, 0.40% Ag, 0.22% As, 0.035% Sb and 0.020% Bi.

The electrolysis was initially effected in a conventional manner. The quantity of electrolyte drained off to reduce the increase of copper content in the solution was held at the lowest possible level in order to preserve as much arsenic as possible in the electrolyte. The following electrolyte composition was obtained at the steady state: 40 g./1. Cu, 70 g./l. H 80 11 g./l. As of which 0.7 g./l. was As(III), 0.40 g./l., Sb of which 0.10 g./l. was Sb(V) and 0.30 g./l. Bi.

The produced cathodes had the following co-analysis calculated in g./t.:

An additional experiment was made with the same anodes and the same electrolyte although a minor quantity of the circulating electrolyte was deflected and passed to a sulphur dioxide adsorption column. The adsorption column comprised a 1500 mm. long glass tube having a diameter of 40 mm. and filled with Rasching-rings. The bottom of the column was provided with means for feeding sulphur dioxide to the column while the deflected electrolyte was fed to the top of the column. The ab- 10 sorption column further comprised a similar glass tube of which the electrolyte treated with the sulphur dioxide was introduced and to which air and steam were charged to strip off the surplus sulphur dioxide.

The apparatus is illustrated in the accompanying drawing, in which a column 1 is provided with an electrolyte 2 at the top of the column. Located at the lower portion of the column is a small container 3 in which gas entrained with the electrolyte is separated. Arranged in the lower portion of the column 1 is a gas distribution filter 4 to which sulphur dioxide is passed through a conduit 5. The column 1 is filled with Raschig-rings 6 and the downwardly flowing electrolyte meets the sulphur dioxide in counter-flow. The column communicates with atmosphere through a conduit 7 and the electrolyte flows from the container 3 up through a conduit 8 and further through a connecting pipe 9 which communicates with atmos phere via a conduit 10 and the conduit 7. The electrolyte is passed from the conduit 9 to a second column 11, similarly filled with Raschig-rings 12. The column is provided at the lower end thereof with a container 13 for separating gas bubbles. Air and steam are passed through the lower portion of the column 11 through a distribution filter 14 via a conduit 15. The electrolyte is removed from the container 13 through a conduit 16 and returned to the electrolytic cell.

The amount of electrolyte passing through the S0 system was adjusted so that the content of As(III) in the electrolyte contained in the electrolytic cell was 3 g./l. To achieve this it was only necessary to pass 2% of the deflected and circulated electrolyte through the SO system. When reducing a part of the deflected stream with $0 the composition of the total amount of electrolyte in steady state was: 40% g./l. Cu, g./l. H 50 8 g./l. As(V), 3 g./l. As(III), 0.20 g./l. Sb, of which less than 0.02 g./l. was Sb(V) and 0.15 g./l. Bi.

The atmosphere at the bath surface of the cells was quite free from S0 odours. Neither could the presence of 50 be shown with Drager-testers.

The produced cathodes had lower contents of antimony, bismuth and arsenic than the aforementioned contents in the cathodes from commercial electrolysis.

Electrolysis of anodes with elevated contents of antimony and/or bismuth gave the same good results. When effecting an electrolysis with anodes containing 1100 g./t. Sb and 800 g./t. Bi the antimony and bismuth contents in the electrolyte was still low and cathodes produced from these anodes had the following co-analysis calculated in g./t.:

As is evident from the above, the total content of bismuth in the cathode copper was reduced by a third and the content of antimony reduced by half as compared with a conventional electrolyte and conventional anodes having a considerably lower content of antimony and bismuth.

In contradistinction to conventional electrolytic processes carried out in the presence of antimony no deposits Were observed in the conduits.

The quantity of S0 required with the two aforementioned tests was 0.55 ton per 1000 tons of cathode copper. As will be evident from the example, the SO -System required is also of small dimensions. It can be calculated that for a complete electrolytic system having a capacity 1 1 of 100,000 ton Cu/year, two columns each of 4 meters in height are required, i.e. an absorption column with an inner diameter of 0.3 m. and a stripper column with an inner diameter of 0.4 in.

EXAMPLE 3 Commercial anodes were electrolyzed in a pilot-plant comprising five cells having three electrode pairs (three anodes and four cathodes) in each cell. The system was equipped with thyristorized current reversal equipment. Each test was carried out at a temperature of 60 C.

The composition of the anodes was: 98% Cu, 0.40% Ni, 0.40% Ag, 0.06% As, 0.07% Sb and 0.02% Bi.

Test l.-Conventional electrolysis A conventional electrolyte was used containing 40 g./l. Cu at the upper edge of the electrodes and 45 g./l. at the lower edge thereof, and 170 g./l. H 80 4 g./l. As, of which 0.5 g./l. was As(lII), 0.5 g./l. Sb and 0.35 g./l. Bi.

Conventional DC current was used during the electrolysis, the highest possible current density without causing troublesome passivation tendencies was 225 a./m. The soft annealing temperature of the produced cathode copper was 188 C.

Test 2.Current reversal with conventional electrolyte Conventional electrolyte was used in this test and the current was periodically reversed, the current being lead in one direction for 100 sec. (toward current) and then in the opposite direction for 6 sec. (back current). It was possible to raise the effective current density (production intensity) by 23-27% with unchanged soft annealing temperature of the product compared with test 1. The limits were set partly by increased intensity of floating slime, which infected the cathodes, and partly by tendencies to anode passivation.

Test 3.Current reversal with electrolyte according to the invention The electrolyte used was produced in accordance with the present invention and contained 46 g./l. Cu at the upper edge of the electrodes and 51 g./l. at the lower edge of the electrodes, and 170 g./l. H 80 3 g./l. As(III) and 11 g./l. As(V), 0.2 g./l. Sb and 02 g./l. Bi. The principle of current reversal was applied, the forward current having a duration of 100* sec. and the back current a duration of 6 sec. The elfective current density (the production intensity) could be increased from 46-50% at the same time as an improved cathode copper quality was obtained (a soft annealing temperature 179 C.).

The tests illustrate that by applying the principles of the present invention it is possible to obtain a highly advantageous electrolytic refining process by a special combination of a new electrolyte composition and current reversal. As will be evident from the examples, a 50% increase in production can be provided for a given cell construction, which implies a particularly significant economic advantage.

By using solely the current reversal principle and a conventional electrolyte it is possible to obtain an increase in production of 23-27%, while when using the electrolyte composition of the invention without the current reversal principle it is only possible to obtain a slight increase in production with retained cathode quality. Thus, it is extremely surprising to one having normal skill in the art that an increase in production of 50% is possible without impairing the quality of the cathode by combining the current reversal principle and the electrolyte of the present invention.

I claim:

1. In the electrolytic refinement of copper anodes containing at least one of the substances antimony and bismuth in quantities exceeding 400 grams per ton with respect to antimony and 200 grams per ton with respect to bismuth, an improvement for avoiding the formation of floating slime comprising using an electrolyte containing at least 3 g./l. of arsenic in the trivalent and pentavalent form and supplying sulphur dioxide to said electrolyte in a quantity sufficient to maintain at least 1 g./l. of said arsenic in the trivalent form.

2. A method according to claim 1, characterized by regulating the quantity of trivalent arsenic in the electrolyte to between 2-5 g./l.

3. A method according to claim 1, characterized by regulating the quantity of pentavalent arsenic in the electrolyte to between 7-15 g./l.

4. A method according to claim 1, characterized by maintaining the quantity of trivalent arsenic by continuously adding trivalent arsenic in quantities corresponding to the consumption thereof during the process of electrolysis.

5. A method according to claim 4, characterized by adding trivalent arsenic in the form of arsenic trioxide.

6. A method according to claim 4, characterized by adding the trivalent arsenic compound to a withdrawn part stream of electrolyte, and then recirculating said part stream of electrolyte.

7. A method according to claim 1, characterized by charging the trivalent arsenic to the electrolyte in the form of arsenic alloyed in the anode copper in quantities such that the total quantity of arsenic released from the anode copper reaches the quantities desired in the elec trolyte.

8. A method according to claim 1, characterized by regulating the total content of arsenic and by regulating the ratio between trivalent and pentavalent arsenic by reducing pentavalent arsenic to trivalent arsenic with sulphur dioxide.

9. A method according to claim 8, characterized in that the reduction is effected by adding sulphur dioxide to the electrolyte.

10. A method according to claim 9, characterized by adding the sulphur dioxide in gas form.

11. A method according to claim 9, characterized by adding the sulphur dioxide through dispersing means located in the lower portion of the electrolytic cell.

12. A method according to claim 9, characterized by adding the sulphur dioxide to a withdrawn portion of the electrolyte, and returning said withdrawn portion of the electrolyte to the electrolysis tank subsequent to the reduction of pentavalent arsenic to trivalent arsenic.

13. A method according to claim 12, characterized by passing the electrolyte treated with sulphur dioxide to a stripper before returning said electrolyte to the electrolytic cell, and removing the surplus of sulphur dioxide from said electrolyte in said stripper.

14. A method according to claim 1, characterized by adjusting the arsenic content of the electrolyte by adding a pentavalent arsenic compound in quantifies to satisfy the desired total content, and reducing the necessary quantity of pentavalent arsenic to trivalent arsenic with sulphur dioxide.

15. A method according to claim 14, characterized by adding the pentavalent arsenic in the form of arsenic pentoxide.

'16. A method according to claim 1, characterized by reducing the consumption of trivalent arsenic in the electrolyte by substantially preventing air from coming into contact therewith.

17. A method according to claim 1, characterized by periodically reversing the current during the electrolysis, said periods being of the order of 1-15% of the total electrolysis period.

18. A method according to claim 17, characterized by maintaining the electrolysis temperature to between 55- 70 C.

13 14 19. A method according to claim 18, characterized by 1,979,229 10/1934 Pitzer. maintaining the electrolysis temperature between 63- 66 C. JOHN H. MACK, Primary Examiner References Cited R. L. ANDREWS, Assistant Examiner UNITED STATES PATENTS 5 657,119 9/1900 Klepetko et al. US

2,742,415 4/1956 Lawrence et a1. 204-108 293 

